sediment characteristics and heavy mineral distribution...

e-Journal Earth Science India, Vol. I (III), pp. 102-118 http://www.earthscienceindia.info/

Sediment Characteristics and Heavy Mineral Distribution in Tamiraparani Estuary and Off Tuticorin, Tamil Nadu-

SEM Studies

M. Suresh Gandhi, A. Solai, K. Chandrasekaran

and V. Rammohan Department of Geology, University of Madras

Guindy Campus, Chennai– 600 025 Email: [email protected]; [email protected]

Abstract The study area Tamiraparani located between latitudes 8° 25΄ and 9° 13΄ N, and longitudes 77° 10΄ to 78° 10΄ E along the south- east coast of India. All samples were collected between Pre-monsoon July, 2003 and Post-monsoon February, 2004. The heavy mineral assemblage of the study region is governed by the distribution of different type of minerals. However, the assemblage is restricted to the dominance of few selective minerals like garnet (colourless), garnet (pink), zircon, rutile, chlorite, etc. From the SEM studies, it is observed that the grains are sub-angular with various surface solution features, rounded crescent like pits, straight net like sutures; v-shaped pits are noticed in pre and post-monsoon samples. The coarser nature of the grains may be due to the influence of river activities. The weathered surface futures also noticed in few samples. The sediment texture, mineral and SEM studies, clearly indicate that in pre-monsoon the erosional activities are predominant than the post-monsoon. In both the periods, sediments transport was from river and estuary towards the beach and marine. In pre-monsoon, the deposited sediment were transported and shifted due to longshore current action. But, in the post-monsoon period the sediments deposited due to the multi-source like riverine and marine influence is observed.

Introduction

Scanning electron microscopy (SEM) plays an increasing role in geology as a tool for understanding the nature of surface morphometric features in sediment samples. During last 15 years quartz grain surfaces have been studied extensively (Kuenen and Perkok, 1962; Krinsley and Donahue, 1968; Margolis, 1968; and Margolis and Krinsley, 1974). These studies have shown that fine surface textures indicate depositional environment and mode of transport. Heiken (1972, 1974) and Walker and Croasdale (1972) studied ash using the SEM and related ash morphology to magma composition and eruption type. Honnorez and Kirst (1976) combined SEM along with optical microscopy to develop a system of morphometric quantification. Huang and Watkins (1976) and Huang et al., (1980) have analyzed angular surface pits on deep sea ash particles in order to estimate the frequency of impacts in volcanic eruption columns of silicic composition.

For beach sediments heavy mineral studies Scanning Electron Microscope (SEM)

is used to verify the general morphological characterizes (Robson, 1884; Mallik, 1986). However, the majority of studies focus on the examination of surface features. Krinsley and Doornkamp (1973) have studied the surface textures of quartz grains over a decade. Tamiraparani basin is one of the major river basins in Tamil Nadu. Even though studies have been carried out on the different aspects of grain size and beach placer distribution in the study area (Cherian, 2003; Angusamy and Rajamanikcam , 2007), but no systematic study has been taken up to understand the sedimentological and heavy mineral variation using SEM studies in this region.

Sediment Characteristics and Heavy Mineral Distribution in Tamiraparani Estuary and Off Tuticorin,

Tamilnadu- SEM Studies: M. Suresh Gandhi, A. Solai, K. Chandrasekaran and V. Rammohan

Study Area

The study area Tamiraparani is located between latitudes 8° 25΄ and 9° 13΄ N,

and longitudes 77° 10΄ to 78° 10΄ E along the south-east coast of India. Study area and sampling locations of Tamiraparani Estuary off south-east coast of India are shown in the Fig.1. Geographic locations of sampling stations are shown in the Table-1 covering Tirunelveli and Tuticorin districts of Tamil Nadu. Tuticorin is geographically located in the Gulf of Mannar. The coastal belt between Vembar to Thiruchenduris is represented by raised beaches with sand bars parallel to the present coastline. The sand bars trend in north-south direction. In the coastal area between Thiruchendur to Manappad, there are sand dune and terri dune complexes. The Tamiraparani and its tributaries originate in the eastern slopes of Western Ghats, the hilly catchment area of which is influenced by both southwest and northeast monsoons.

Fig.1: Map showing sampling locations in the study area.


Table-1: Geographical locations of Tamiraparani River, Estuary and Marine sediment samples

Location Longitude Latitude Location Longitude Latitude

R1 78° 00' 07.17''

08° 37' 16.94''

M1 78° 08' 27.44''

08° 40' 03.23''

R2 78° 01' 41.40''

08° 37' 16.94''

M2 78° 09' 29.80''

08° 40' 15.17''

R3 78° 03' 01.78''

08° 37' 15.55''

M3 78° 10' 28.01''

08° 40' 08.78''

R4 78° 04' 36.01''

78° 04' 36.01''

M4 78° 11' 37.30''

08° 40' 03.23''

R5 78° 05' 07.89''

08° 38' 15.14''

M5 78° 12' 49.36''

08° 40' 17.09''

R6 78° 07' 41.71''

08° 35' 52.41''

M6 78° 14' 12.50''

08° 40' 10.16''

R7 78° 07' 12.61''

08° 35' 59.33''

M7 78° 15' 30.11''

08° 40' 03.23''

R8 78° 07' 07.07''

08° 36' 29.82''

M8 78° 16' 51.59''

08° 40' 10.16''

R9 78° 07' 22.31''

08° 36' 51.99''

M9 78° 18' 11.97''

08° 40' 04.62''

R10 78° 07' 19.54''

08° 37' 23.87

M10 78° 18' 13.35''

08° 35' 51.02''

R11 78° 07' 20.92''

08° 38' 04.06''

M11 78° 16' 41.89''

08° 35' 42.71''

R12 78° 06' 49.05''

08° 37' 39.11

M12 78° 15' 18.74''

08° 35' 28.85''

R13 78° 06' 04.71''

08° 38' 06.83''

M13 78° 14' 13.61''

08° 35' 53.79''

R14 78° 07' 32.01''

08° 38' 38.70''

M14 78° 13' 15.41''

08° 35' 33.00''

R15 78° 06' 50.44''

08° 38' 33.16''

M15 78° 12' 04.73''

08° 35' 48.25''

R16 78° 05' 56.39''

08° 38' 53.94''

M16 78° 10' 38.81''

08° 35' 55.18''

R17 78° 07' 02.91''

08° 39' 09.19''

M17 78° 09' 40.61''

08° 36' 07.65''

R18 78° 06' 51.82''

08° 39' 41.06''

M18 78° 08' 35.48''

08° 35' 44.09''

Methodology

Samples were collected from selective localities like river, estuary and marine for SEM studies. Three size fractions were chosen from sieved samples to represent each sample. Sizes used were fine, medium and coarse in order to study size dependency of textures. The micro-features of grain size for sediment characteristics and heavy minerals like illmenite, garnet, etc. from the study area are attempted using Scanning Electron Microscope (Model; JEOL JSM 6360). After washing, samples were mounted on stainless steel stubs using double-stick tape. Approximately (5 grams) 20 grains from each of the three size categories were mounted on a single stub pasted with double side adhesive tape.

Samples were coated with platinum-palladium (JFC 1600) in order to counteract

grain surface charging while scanning with the electron beam. Both whole grain and detailed surface photographs were taken using the secondary electron mode at 20 keV.



For heavy mineral SEM studies, selective minerals from both the monsoons were picked and photographed.

Result

The Table-2 shows the mechanical and chemical features of the selected grains with respect to river, estuary and marine samples.

Table-2: Morphological features for the grains in river, estuary and marine sediments in

pre and post monsoon- (Approximately for 0.25 grams)

M E C H A N I C A L F E A T U R E S

Surface

Features

River

Pre-

Estuary

Pre-

Marine

Pre-

River

Post-

Estuary

Post-

Marine

Post-

V- shaped pits 5 4 2 4 3 3

Rounded-crescent like pits

11 12 4 2 8 5

Straight net like sutures

21 0 2 8 0 2

Conchoidal Fracture (<10 micron

18 4 4 12 6 18

Conchoidal Fracture (>10 micron

21 3 2 5 4 14

Arcuate Step like Furrows

2 0 0 3 0 2

Dish Shaped depressions

6 4 2 2 4 8

Upturned Plates 0 0 0 0 0 0 Crevasses 2 4 12 4 4 18

Etch V’s 4 0 0 2 0 4 C H E M I C A L F E A T U R E S

Crystalline overgrowth 0 0 0 0 0 0 Silica Precipitation 2 4 0 0 0 0

Silica Flowers 0 0 0 0 0 0

Silica Globules 0 0 0 4 0 0 Solution Pits 12 14 4 2 12 84 Surface Solution Features

0 2 4 0 11 96

Triangular Oriented Pits

0 0 2 2 4 4

Block Removal 0 0 0 0 0 0


Granulometric Surface Feature Studies

Pre-Monsoon samples: The grains are sub-angular with various surface solution features is observed in

pre-monsoon river samples. Rounded crescent like pit, straight net like sutures, V- shaped pits are noticed. The coarser nature of the grains may be due to the influence of river activities. The weathered surface futures also noticed in few samples. The estuary grains shows more or less similar pattern as of river samples. This indicates that the sediments were transported and deposited in the estuaries. The quartz grains show the conchoidal fractures. The grains are irregular in shape. In pre-monsoon marine samples, the grains are again sub-angular to irregular in nature. Few cracks like features also noticed. This may be due to mechanical activities of waves and currents. Further, due to erosional activities the grains surfaces are smoothless in nature (Plate I-II).

Post-Monsoon samples:

The river samples show sub-angular, angular and irregular grains. Numerous pits

are observed in the surface of the few grains which may be due to chemical and mechanical disintegration. In comparison to pre-monsoon samples, the post-monsoon samples are less disturbed by mechanical activities. In estuary, the grains are angular to sub-angular in nature. Numerous pits and weathered samples are noticed which may be due to the depositional activities. In marine samples fossil like preservations were also observed. The preservations like v-shaped pits, grooves are noticed in these samples. Few rounded grains are also observed which may be due to long transportation (Plate III-IV). It is clearly indicated that the depositional activities are predominant in post-monsoon compare to pre-monsoon.

Heavy Mineral SEM Studies

The Fig. 2 shows the distribution of total percentage of heavy mineral in Tamiraparani River and off Tuticorin. From the study of heavy mineral assemblages, it is observed that the non- opaque percentage is 3.26 to 5.47% in marine samples and opaque raging form 3.67 to 5.72%. Over all the higher percentage is noticed in the river samples may be due to the fast moving action of wind and water. In estuary, the heavy mineral distribution is lesser (0.8% to 11.6%) due to the movement of sediments from river and the erosional activities. The Plate- V shows the SEM photographs of the selective minerals. Illmenite:

Illmenite grains show varied features. Most of the grains are sub-angular to

rounded with moderate relief. Under the higher magnification a number of oriented crescentic pits are noticed from solution cavity (PLATE-V). A long residence time allowed the grain to develop this type of features. Some grains shows development of undulatory wavy surfaces probably because of mechanical reaction and removal of large blocks. Impact ‘V’ marks are also noticed which are some times modified by etching. Spit like features also observed in some minerals. This indicated that the grains must have longer residential time in the basin. According to Mallik (1986) the most important features that reflect the sub-aqueous environment are the V- shaped pits. This feature is observed in the illmenite near Tuticorin zones.



PLATE-I: Fig.1: Angular to sub-angular with mixed grains Fig.2: Conchoidal fractures like quartz Fig.3:V-marks and pitting at edges Fig.4: Solution pits and minor cavities Fig.5-6: Mechanical breakages at the edges Fig.7: Sub- rounded with small pits Fig. 8: Depressions with sharp edges Fig.9 -10: Precipitations with pits Fig.11: Sharp edges Fig.12: Cracks and fractures Fig.13-14: Cracks with depressions Fig.15: Triangular pits Fig.16: Sub-angular mixed grains Fig.17: Solution pits and precipitation with sub angular grains Fig.18: Precipitations structures inside the depressions Fig.19: Rectangular solution pits Fig.20: Fibrous needle with cracks


PLATE-II: Fig.1: Angular with overgrowth grains Fig.2: Solution pits and depressions due to chemical activities Fig.3: Surrounded grains Fig.4: Silica precipitates with sub angular Fig. 5: Sub-angular with shallow depressions Fig.6: Conchoidal fracture with sharp edges Fig.7-8: Suture surface with evenly cut cleavages Fig.9 -10: Long scratch with elongated cleavages Fig.11: Deep depressions Fig.12: Fibrous like straight cleavages Fig. 13:Conchoidal fracture Fig. 14-16: Uneven and sub angular to angular grains Fig. 17-19: Sub-angular grains Fig. 20-21: Solution pits and precipitation enlarged view



PLATE-III: Fig.1: Sub-angular grains Fig.2: Depressions at the surface Fig.3: Sharp angular grains with pits Fig.4: Solution pits Fig. 5-6: Over growth and elongated grains Fig. 7: Sharp and angular grains Fig.8-9: Conchoidal fractures Fig.10: V-shaped depression and fractures Fig.11: Sub-angular grains Fig.12: Depressed edges Fig.13: Precipitations and cracks Fig.14: Straight factures, Fig. 15: Sub- rounded grains Fig. 16: Small depression sat the edges, Fig. 17: Solution pits due to chemical activities Fig. 18: Large Depressions due to solution affects Fig. 19: V-shaped depressions Fig.20: V-shaped depression with precipitations Fig.21: Dish shapes cavities Fig.22: Straight fractures with depressions Fig.23: Straight fractures Fig.24: Depression filled with precipitations


PLATE-IV: Fig.1: Sub-angular to sub rounded Fig. 3: Small pits due to solution Fig. 4: Crakes with sub angular grains Fig. 5-7: Conchoidal fracture with solution pits Fig. 8: Depressions with solution pits Fig.9-10: Sub-angular grains Fig.11: Sub- angular to surrounded grains Fig.12: Angular with fossil like impression Fig.13: Diatoms like impression? Fig. 14-15: Sub- rounded to sub- angular grains Fig. 16: Disc shaped with vertical fractures Fig. 17: Elongated triangular grains Fig. 18: Step like furrows Fig.19: Sub- angular grains Fig.20-21: Solution pits due to chemical activates



The rounded edges of illmentite and mechanical abrasion suggest their prolonged transportation and the solution pits indicate their longer residential time in the basin. However, the illmenite of this area shows the features produced due to mechanical breaking. These are with irregular or regular breaking along or across cleavage plane, abrasional marks, mechanical ‘V’ marks, etc. Most of the illmenite grains are sub-angular to sub-rounded in shape. These mechanical features indicate illmenties are being transported in aqueous medium and they were not subjected to chemical processes in this region. Krinsley and Droonkamp (1973) have also reported these characteristic features for the paleo-sediments. Similarly, Gujar (1996), Mallik (1986) and Cherian (2003) have reported the same features off South Konkan coast, Kerala beach and Valinokkam – Tuticorin sector respectively.

Fig.2: Heavy Mineral Distribution of Pre-monsoon and Post-monsoon of Tamiraparani

River, Estuary and Marine samples


PLATE-V: Fig.1: Illmenite filled with precipitations Fig.2: Sub-rounded illmenite Fig.3: Illmenite with small pits Fig.4: Illmenite with surface cracks Fig. 5-6: Garnet with conchoidal fracture Fig.7: Garnet with cracks Fig. 8: Garnet with deep pits and V- shaped pits Fig. 9: Rutile with subangular to rounded Fig.10: Sub-rounded rutile with solution activities Fig. 11-12: Elongated rutile Fig.13: Sillimanite with crystal faces Fig.14: Sillimanite with solution pits Fig.15: Broken sillimanite with crystal faces Fig. 16: Sillimanite with triangular pits at the edges Fig.17: Sub-rounded zircon Fig.18: Zircon with surface depressions Fig.19: Elongated zircon with deeper depression by chemical action Fig. 20: Elongated zircon



Garnet:

Garnet grains are angular to sub-angular with moderately high relief. Some

angular grains show conchoidal fracture with embayments formed due to solution effect. As a result of etching, large depressions have formed with in which precipitation structures are identified.

In most of the grains impact features and etch marks prevail which designates

considerable physical activity in the nearshore zone. The rounding has been caused mainly by transportation that, in turn, indicates polycyclic nature of sediments and change in energy conditions.

Zircon:

The minerals are euhedral, broken, rounded in nature. Few varieties show well-

developed crystal faces. A deep and shallow depression also noticed in zircon, may be due to various transport mechanism.

Sillimanite:

The minerals are elongated in nature. Well developed crystal faces are noticed. Triangular pits are also observed. This indicates the mechanical actions taken place due to transportation. Rutile:

The rutiles are sub-rounded in nature. Due to long transportation some grains obtain rounding. It is frequently distributed along the investigated area.

Light Mineral SEM Studies

Quartz and feldspar are the principal light minerals present, among which quartz occupies the major part followed by orthoclase and plagioclase feldspar.

Quartz is characterized by low relief, absence of perfect cleavage, low order

interference color. The SEM studies of quartz grains are applied to understand the post-depositional or digenetic history of the sediments (PLATE-III &IV) During the processes of transportation and deposition, various micro-features are developed by mechanical and chemical processes and are influenced by physical and chemical properties of grains such as hardness, cleavage, solubility, tenacity, etc.

Based on mode of formation, Rahman and Ahmed (1996) have classified various

micro-textures into four groups. They are, (i) Mechanical formation of conchoidal fracture, pitted surface, etc., (ii) Mechanical/ chemical process- these are responsible for textures like angular

outline, relief features, V form etc., (iii) Dissolution chemical process- generates cavities, solution pits, etc., (iv) Precipitational chemical process- forms silica, precipitation and crystalline

outgrowths.

The common features observed are conchoidal fractures with cavities and solution pitting, chemical etching marks. In the present study, the quartz grains of the marine and beaches show both conchoidal fractures and etched marks indicating high- energy environment as well as the longer stay of sediments in the depositional basin.


Grain morphology

Few textural features were chosen to describe surfaces of the grain. These features constitute the most common surface morphologies of different mineral grains determined after reconnaissance observations and considerations of the extensive work done with grain surfaces by Margolis and Krinsley (1974). The essential characteristics identifiable with each micro-feature are briefly discussed. Heiken (1972) and Honnorez and Kirst (1976) described the concept of overall grain morphology with respect to basaltic ash. For our study selected grains were studied and photographed.

Conchoidal Fractures:

These are distinguishing characteristics of glassy materials and quartz, and result from brittle deformation due to compressive contact between two surfaces. In the beach environment this situation occurs during transport from different environments. Conchoidal fracture patters vary from regular dish shapes (uncommon) to irregular elongate fan-like or trough-like depressions. The common parallel step-like fractures that curve around the conchoidal depression are thought to be expressions of planes of weakness in the glass similar to cleavage planes in crystals (Margolis and Krinsley, 1974). These featherlike fractures that produce a rippled surface texture may also be produced by acoustic wave phenomena (Kragelskii, 1965). Elongate conchoidal fractures are most evident on grain edges. The conchoidal fracture are shown in pre-monsoon samples (PLATE- I, II) and few in post- monsoon samples in our study region.

V-Shaped Depressions:

These are micropits which vary from triangular depressions to elongate grooves that widen in one direction. Chemical effects as well as abrasion quickly obscure these features. Many of the V-shaped depressions may be only several microns in maximum dimension. These are only evident under magnification of several thousand times. Two processes can account for the V-shaped depressions: 1) tangential impacts with sliding of one grain over another (Lawn and Wilshaw, 1975), and 2) chemical solution in areas of localized order or microlite development. Huang et al. (1980) considered them to be the result of glancing impacts. They appear to form on all types of tephra particles. In our study region, V-shaped depressions are noticed more in post- monsoon samples (PLATE- III-IV). Upturned Plates:

These features are visible only at magnifications above about 1,000 X arrow. Plates resemble sub-parallel ridges that are slightly raised above the general surface. They form ridge like structures that are generally less than 10 µm in long dimension. Plate edges are smooth and generally of equal height due to solution/precipitation effects. Plates are found near edges of grains, especially on exposed surfaces as opposed to semi-protected hollows (Plate-I Fig. 8. depressions with sharp edges). Krinsley and Smalley (1973) have suggested that upturned plates are oriented along traces of cleavage planes in quartz and likely are cleavage plates. Their formation is due to failure along planes of weakness due to mechanical stress applied during impact or crushing.

Grooves:

Grooves include elongate scratches and troughs that may be slightly curved. These grooves are oriented in a preferred direction, occur with conchoidal fracture, and appear in sets. Grooves are, however, relatively uncommon on the samples studied.



Cracks:

Cracks are a mainly due to mechanical process, and are straight or slightly curved. Separation along cracks generally is less than 10 µm. Cracks are best developed on vesicle surfaces and may radiate from equal angles in groups of two or four. Overall, these features appear similar to mud cracks and, when they intersect, form polygonal plates on surfaces; they appear to form both before and after conchoidal fracture. Cracks that project through a grain may be due to the thermal stress of quick cooling or by impact with another grain or surface. Cracks in the grain skin could be due to grain expansion after formation of a brittle skin or contraction of the hydrated skin often followed by accumulation of alteration materials. The grooves and depression are more in pre- monsoon samples on our study region than the post- monsoon (Plate- I, Fig. 12-14). Chemical Alteration:

Grains are sugary in appearance when totally altered; however, they retain their vesicular morphology. Solution and precipitation occur together on the same grain. Resulting textures include pitted or scalloped surfaces on a micron scale, rounding of upturned plates or other sharp features, and development of a frosted, light diffused surface as compared to the vitreous surface of fresh glass. Solution pits and Silica precipitation are more in our samples (Plate - III, Fig. 17. Solution pits due to chemical activities) in all the monsoons; this may be due to chemical alteration.

Discussions

The study mainly focused on the nature of sediments and its characteristics with respect to grain size and minerals distributions using SEM studies in pre- monsoon and post- monsoon of Tamiraparani and adjoining regions. In the Tuticorin samples, the sediments are medium grained; nevertheless, they are better sorted as a result of the prevailing high wave energy conditions there. At Tuticorin itself, however, the sediments are poorly sorted, even though their mean size is finer. The growth of the spit at Tuticorin impedes the movement of sediment-laden littoral currents; as a result, lateral updrift causes sediment deposition, especially in the northern part of the Tuticorin sector. In this process, the materials removed from the Tamiraparani River mouth are immediately deposited in the Tuticorin region, and no further sorting is possible. Besides the above, the deposition of fine sediments from the adjacent reefs may be a further factor causing the poor sorting of sediments. The better sorting at the remaining stations is probably due to the prevalence of wave convergence throughout the year and the finer size of the sediments.

The post-monsoon of near by Tuticorin region has been identified as zone of

erosion. Due to more wave energy updrift alongshore current whereas, in Tuticorin, the sediments in this marine region are medium to fine sand but only medium sand in river and estuary samples. The beach slope is steep and the width is less. The long shore current direction is NE/ SW direction. But, in Tuticorin, the zone of accumulation is taking place. Here the wave energy is decreased and suspended sediment transport become predominant. Due to the accumulation of sediments, it is identified a leaned zone of heavy mineral concentration. In river and estuary samples the sediments are medium sand and moderately sorted in nature.

In the marine samples the nature of sediment is medium-to-fine, moderately-to-

well sorted and positive symmetrically skewed sediments indicates, probably as a result of the influence of palaeo-sediments deposited by rivers from inland as well as by waves and currents from offshore. This fact is supported by Angusamy and Rajamanickam (2007). Further, in the post-monsoon the estuary sediments are medium to poorly sorted with positive symmetrical and fine skewed indicates the depositional process. Further, it indicates the deposits of near source region and marked from river Tamiraparani.


The mean size of the post-monsoon sediments on the marine region indicates the

predominance of very fine sediments with an admixture of medium-grained sands. The preponderance of such fine sediments is probably due to the deposition and indicating the influence of aeolian activities in transporting fine sediments in suspension and of saltation from the adjoining coastal landforms as well as from estuaries.

In pre-monsoon, the sediments are medium to fine, moderately sorted and fine

skewed indicates the erosional activities. Similar type of observations has been made by Cherien (2003). The erosion may be due to onshore and offshore movement of material. In Tuticorin zone, the erosion may be due to the northward movement of sediments by longshore current. The beach morphology shows that during pre-monsoon the beach width is only 16m whereas in post-monsoon it is 26 m. The beach slope is 4o and 7 o in post- and pre- monsoon respectively.

The distributions of heavy minerals are higher in river and marine sediments than

in estuary in pre-monsoon compare to the post-monsoon. This may be due to erosional activities and high winnowing actions in pre-monsoon, the heavy mineral enrichment is favorable in marine sediments and in depositional periods of post- monsoon, the distribution of heavy minerals are lesser in amount may be due to long shore current actions the deposition of heavy are transported to northern side.

The predominance of garnet populations in beach, dune marine and river sands

from Tuticorin, southern India, which almost exclusively comprise low grossular high-pyrope garnets. These are derived from the high-grade (granulite facies) metasedimentary and charnockitic rocks that form the basement in this area. During the monsoon periods, the lesser percentage of topaz, glucophane, actinolite, sillimanite and kyanite helps to infer that the Tamiraparani river sediments may not be reaching these beaches in significant proportion (Loveson, 1994). Hence, in the estuary the distribution is lesser in amount in both the monsoon. Similar studies on the sediments of Vaippar River (Udayaganesan, 1993) as well as the beaches of northern Tamilnadu (Muthukrishnan, 1993; Mohan, 1995) have shown the negligible contribution of heavy minerals by the rivers to the respective beaches. The presence of strongly etched chlorites and garnet in this zone suggests either a longer distance transportation of sediments or a longer stay of sediments in the depositional basin.

Conclusion

SEM studies of placer minerals indicate that a number of micro-textures are developed by mechanical and chemical processes influenced by physical properties mainly cleavage. The SEM studies support that the sources of sediments are from crystalline rocks either from igneous - metamorphic rocks or paleosediments, that too from mixed sources from littoral currents and riverine and beach. The SEM study shows that in pre- monsoon, the morphological changes like pits, grooves, depressions etc. are lesser than the post- monsoon grain samples. The river and estuary grains show more or less similar patterns in both the monsoons. In the present study the quartz grains of the marine and beaches show both concordant fractures and etched marks indicating high – energy environment as well as the longer stay of sediments in the depositional basin.

Acknowledgement: We are thankful to Professor S.P. Mohan, Head, Department of Geology, University of Madras for providing SEM facilities and constant encouragement during this work. The first author wishes to express his gratefulness to Prof. V. Rajamanickam, Dean, Department of Disaster Management, SASTRA University, Thanjavur & former Head, Department of Earth Sciences, Tamil University, Thanjavur for interpretation in the Heavy mineral studies.



Reference

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